The present invention relates to a focus detection apparatus for use in an optical apparatus such as a digital camera and a video camera.
Conventional focus detection of an image-taking optical system in a digital camera or the like is realized with a contrast detection method using an image-pickup device. Typically, the focus detection of such a contrast detection method presents the problem of taking a considerable time period for focus adjustment to achieve focusing since the highest contrast is found while an image-taking optical system is moved little by little on an optical axis.
To avoid the problem, focus detection is often performed with a phase difference detection method, for example in a digital camera on which a lens apparatus is removably mounted.
Since the focus detection of the phase difference detection method allows the determination of a defocus amount of an image-taking optical system, it has the advantage of significantly reducing the time taken to achieve focusing as compared with the contrast detection method.
The focus detection of the phase difference method includes vertical line detection in which focus detection is performed for an object having a contrast component in a horizontal direction and horizontal line detection in which focus detection is performed for an object having a contrast component in a vertical direction. To handle various objects, focus detection is performed by combining the vertical line detection and the horizontal line detection on an image-taking screen. In addition, focus detection of a cross type is performed in which the vertical line detection and the horizontal line detection are made at the same position on an image-taking screen.
In recent years, proposals have been made for multipoint focus detection in which a plurality of focus detection areas are provided for performing the vertical line detection and the horizontal line detection on an image-taking screen and area-type focus detection in which focus detection is performed in continuous areas over a wide range. As one of those focus detection methods, a prior art has been disclosed in which the area-type focus detection and the cross-type focus detection are performed (for example, see Japanese Patent Laid-Open No. 9-184965).
In the prior art, the abovementioned focus detection is applied to a camera on which a lens apparatus is removably mounted. FIG. 19 shows a section view of the center of the camera. In FIG. 19, reference character L shows the optical axis of an image-taking optical system. Although not shown, the image-taking optical system is disposed on the optical axis L to the left of FIG. 19. Reference numeral 2 shows a primary image-forming plane of the image-taking optical system. A main mirror 3 and a sub mirror 4 are placed in front thereof.
The main mirror 3 and the sub mirror 4 are moved out of image-forming luminous flux by a well-known quick-return-mechanism when images are taken. On the other hand, they are held at the positions shown in FIG. 19 when focus detection is made. Luminous flux for use in focus detection passes through a half mirror portion formed near the center of the main mirror 3 and is reflected downward by the sub mirror 4. Reference numeral 5 shows a primary image-forming plane of a focus detection optical system formed by the sub mirror 4, which is optically equivalent to the primary image-forming plane 2.
Then, the optical path is turned by a first flat mirror 6 and passes through an infrared cut glass 7, an aperture 8, and a secondary image-forming lens 9. The optical path is then turned downward by a second flat mirror 10 and finally directed to a focus detection sensor 11. The focus detection sensor 11 is formed of a cover glass and a sensor chip placed near a secondary image-forming plane.
The sub mirror 4 is formed along part of an ellipsoidal plane formed by rotating an ellipse around a central axis A as shown by a dotted line in FIG. 19. One of the two focal points of the ellipse is set to the exit pupil of the image-taking optical system. The other is set to a point B which is an intersection of the central axis A and the optical axis L turned by the sub mirror 4.
The point B is located such that the equivalent air distance from the central point of the focus detection aperture 8 to the first flat mirror 6 is equal to the air distance from the point B to the first flat mirror 6. Thus, the sub mirror 4 holds the aperture 8 and the exit pupil of the image-taking optical system in an image-forming relationship from the basic nature of the ellipse. In other words, it serves as a known field lens in the focus detection of the phase difference method, and the sub mirror 4 and the aperture 8 function as a pupil dividing means. It is possible to direct a plurality of luminous fluxes divided on the exit pupil of the image-taking optical system toward the focus detection optical system by setting appropriate openings in the aperture 8.
FIG. 20 shows a plan view showing the aperture 8 and the secondary image-forming lens 9 viewed from the infrared cut glass 7. Since the secondary image-forming lens 9 is hidden by the aperture 8, it is shown by dotted lines. The aperture 8 has a pair of openings 8-1 and 8-2 and a pair of openings 8-3 and 8-4. The secondary image-forming lens 9 has a pair of lens portions 9-1 and 9-2 and a pair of lens portions 9-3 and 9-4 corresponding to the respective openings.
Thus, of the luminous flux passing through the exit pupil of the image-taking optical system, luminous fluxes divided vertically by the openings 8-1 and 8-2 and luminous fluxes divided horizontally by the openings 8-3 and 8-4 are focused by the secondary image-forming lens 9 which has the four lens portions. Four optical images are formed on the secondary image-forming plane. The focus detection sensor 11 detects a phase difference in the four optical images associated with defocus of the image-taking optical system to realize the focus detection of the phase difference method.
FIG. 21 is a plan view showing the sensor chip of the focus detection sensor 11 viewed from the second flat mirror 10. The sensor chip has four sensor areas formed thereon corresponding to the four lens portions of the secondary image-forming lens 9, in which the lens portions 9-1 and 9-2 correspond to sensor areas 11-1 and 11-2, and the lens portions 9-3 and 9-4 correspond to sensor areas 11-3 and 11-4, respectively. Optical images projected on the sensor areas 11-1 and 11-2 are formed by the luminous fluxes passing through the openings 8-1 and 8-2, respectively, that is, the luminous fluxes divided vertically on the exit pupil of the image-taking optical system, so that the optical images are moved vertically in association with defocus of the image-taking optical system.
Thus, a phase difference in the optical images can be detected by vertically arranging pixels in columns in the sensor areas 11-1 and 11-2. Similarly, in the sensor areas 11-3 and 11-4, pixels are arranged horizontally in rows. In the sensor areas 11-1 and 11-2, the horizontal line detection is performed since an object having a contrast component in the vertical direction can be best detected. In the sensor areas 11-3 and 11-4, the vertical line detection is performed.
Since the aperture 8 is projected by the sub mirror 4 on the exit pupil of the image-taking optical system, the openings in FIG. 20 enlarged at a predetermined magnification show areas on the exit pupil through which luminous flux passes on the exit pupil. Supposing FIG. 20 is already enlarged at a magnification for projection on the exit pupil, the distance between the median points of the openings 8-1 and 8-2 represents the length of a baseline for the horizontal line detection, while the distance between the median points of the openings 8-3 and 8-4 represents the length of a baseline for the vertical line detection.
The circumscribed circle around the openings 8-1 and 8-2 represents the diameter of the exit pupil in which focus detection can be performed in the horizontal line detection, while the circumscribed circle around the openings 8-3 and 8-4 represents the diameter of the exit pupil in which focus detection can be performed in the vertical line detection. In the latter exit pupil diameter, focus detection can be performed in both of the vertical line detection and the horizontal line detection. In other words, the horizontal line detection is performed only in the smaller exit pupil diameter, while both of the vertical line detection and the horizontal line detection are performed in the larger exit pupil diameter. For example, only the horizontal line detection is performed with an F number of 5.6, and both of the vertical line detection and the horizontal line detection are performed with an F number of 2.8.
FIG. 22 shows the respective sensor areas reversely projected in an image-taking range on the primary image-forming plane 2. In FIG. 22, warping due to distortion is ignored. Focus detection areas 13 and 14 are present in the image-taking range 12. The focus detection area 13 is obtained by reversely projecting the sensor areas 11-land 11-2. Since the resulting areas almost match, they are shown as the focus detection area 13. Similarly, the focus detection area 14 corresponds to the sensor areas 11-3 and 11-4. Thus, the focus detection area 13 represents the region in which the horizontal line detection is performed, while the focus detection area 14 represents the region in which the vertical line detection is performed. The shaded area in FIG. 22 representing their overlapping corresponds to the area in which the cross-type focus detection is performed.
As described above, in the prior art, the focus detection of the phase difference method is performed over the wide area and the cross-type focus detection is realized in part of the area.
In general, to define a wider area of the image-taking range as the focus detection area, more luminous flux needs to be directed to the focus detection optical system. For that purpose, it is necessary to place the sub mirror 4 as close as possible to the primary image-forming plane 2 and to increase the light reflecting area of the sub mirror 4. This causes the primary image-forming plane 5 of the focus detection optical system to shift closer to the sub mirror 4 as shown in FIG. 19.
In a focus detection optical system using a known field lens, it is necessary to set the field lens and a field mask near the primary image-forming plan 5 of the focus detection optical system. These members are put into image-taking luminous flux, so that a mechanism for retracting them is required. However, in the prior art, the sub mirror 4 is formed along part of the ellipsoidal surface and is provided with the function of the pupil dividing means, which eliminates the need to provide the field lens. The sub mirror 4 also serves as the field mask when it is formed such that light is not reflected in any area of the sub mirror 4 other than the area necessary for focus detection.
For these reasons, the sub mirror 4 can be easily increased in size in the prior art. As a result, the focus detection of the phase difference method is realized over the wide area.
The prior art, however, has the following problem. Specifically, the relatively wide area can be ensured for performing the horizontal line detection in the image-taking screen, but the area for the vertical line detection is limited to near the central portion. The reason thereof will hereinafter be described.
To provide the area for the vertical line detection as large as the area for the horizontal line detection, it is contemplated that the sizes of the sensor areas 11-3 and 11-4 for the vertical line detection are increased to be about the sizes of the sensor areas 11-1 and 11-2. In the image-taking optical system, vignetting is present, so that the diameter of the exit pupil for use in focus detection changes according to the size of the focus detection area. In general, the diameter of the exit pupil is smaller as the focus detection area is larger, that is, the image height is larger.
To increase the size of the focus detection area for the vertical line detection in the prior art, it is necessary to reduce the interval between the openings 8-3 and 8-4 projected on the exit pupil by the sub mirror 4 in order to support the smaller diameter of the exit pupil. The interval between the lens portions 9-3 and 9-4 and the interval between the sensor areas 11-3 and 11-4 are also reduced accordingly. However, the sizes of the sensor areas 11-3 and 11-4 need to be increased to be about the sizes of the sensor areas 11-1 and 11-2. When the sizes of the areas are increased to reduce the interval between them, the respective sensor areas interfere with each other as apparent from FIG. 21.
In the prior art, the optical images are projected on the sensor areas 11-3 and 11-4 with the same size of the optical images projected on the sensor areas 11-1 and 11-2. Thus, the sensor areas 11-3 and 11-4 are placed at a certain distance from the sensor areas 11-1 and 11-2 in order to prevent interference between the respective optical images. When the interval between the lens portions 9-3 and 9-4 is reduced, the resulting optical images interfere with each other. For those reasons, the focus detection area for the vertical line detection can only be set near the central portion of the image-taking screen in the prior art.